The risks incurred by the population when metals in the form of traces are found in aqueous discharge liquors from chemical, petrochemical, agrochemical, pharmaceutical, plastic or metallurgical plants, and the like, are now known. Mention may be made, for example, of diseases which can result from the ingestion by man of traces of lead (saturnism), of cadmium (proteinuria, itai-itai in Japan), of aluminum (Elsheimer), of mercury (Minamata disease in Japan), of chromium (VI) (cancer), and the like.
It is therefore advantageous to find methods which make it possible to remove these metals in the trace and ultratrace form in wastewater and generally in water. Legislation, in particular European legislation, is particularly strict and gives increasingly lower values for metal contents. For example, the tolerated contents in aqueous industrial discharges are all less than 1 ppm and, in drinking water, this content must not exceed 50 ppb for lead or chromium and 5 ppb for cadmium and must be less than 1 ppb for mercury.
Current processes for removing metals from aqueous effluents involve precipitation in the form of hydroxides or of sulfides, coprecipitation with aluminum, iron or other salts, adsorption on inorganic or organic charges, or ion exchange.
These processes involve physisorption methods or ion exchange methods and relate only to one type of ion and, furthermore, are reversible.
Finally, these processes are generally ineffective for the removal of contaminants present in the form of traces, for example of the order of 1.1 000 to 10 000 ppm.
Thus, there exists a demand for an efficient method for removing metal contaminants present in the form of traces in aqueous effluents.